Process of friction stir welding on tube end joints and a product produced thereby

ABSTRACT

A process of producing shell and tube heat exchangers where the ends of the tubes are secured to a tube sheet while reacting applied FSW forces without introducing a crevice or local deformation near the ends of the tubes. In particular, an interference fit is used to lock the ends of the tubes into the tube sheet without flaring or expanding the tube ends. A FSW process is then used to weld the ends of the tubes to the tube sheet.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/202,636, filed on Mar. 10, 2014, entitled “PROCESS OFFRICTION STIR WELDING ON TUBE END JOINTS AND A PRODUCT PRODUCEDTHEREBY,” which claims the benefit of U.S. Provisional Application No.61/777,438, filed on Mar. 12, 2013, entitled “PROCESS OF FRICTION STIRWELDING ON TUBE END JOINTS AND A PRODUCT PRODUCED THEREBY,” thedisclosures of which are hereby incorporated herein by reference intheir entireties.

TECHNICAL FIELD

This disclosure relates to friction stir welding on heat exchangers,such as shell and tube heat exchangers.

BACKGROUND

Shell and tube friction stir welded (FSW) heat exchangers have beendeveloped for marine grade applications such as ocean thermal energyconversion, thermal desalination and other relatively low temperatureprocesses. In addition, the FSW process is being used in other heatexchangers that operate under higher temperatures and 15 pressures. Inthe FSW process as applied to shell and tube heat exchangers, a solidstate welding or stirring process is used wherein the ends of tube wallsare “stirred” into surrounding tube sheet material without introducingdissimilar metals and without adverse effect to metal grain structure.

In the heat exchanger, the tubes are inserted into tube sheets on eitherend of the 20 bundle of tubes in a manner similar to traditional shelland tube heat exchanger designs. The ends of the tubes are normallyflared prior to the FSW process. This flaring or tubeend expansionallows the tubes to stay in place while reacting the forces appliedduring the FSW process.

The FSW process eliminates crevices that would ordinarily exist between25 mechanically rolled tube ends and the surrounding tube sheetmaterial. Elimination of crevices is desirable to obtain a heatexchanger that can have long-life in a corrosive seawater environment.However, the process of flaring or expanding the ends of the tubes cansometimes introduce an undesirable crevice or locally deformed zone thatcan become a site for preferential crevice corrosion, particularly in aseawater environment.

SUMMARY

This description describes an approach to producing shell and tube heatexchangers, where the ends of the tubes are secured to a tube sheetwhile reacting applied FSW forces without introducing a crevice or localdeformation near the ends of the tubes. In particular, an interferencefit is used to lock the ends of the tubes into the tube sheet withoutflaring or expanding the tube ends. A FSW process is then used to weldthe ends of the tubes to the tube sheet.

The tubes and the tube sheets can be made of any metals commonly used inshell and tube heat exchanger assemblies including, but not limited to,aluminum, carbon steel, stainless steel, titanium, copper or othermetals and alloys thereof.

The interference fit can be achieved in any suitable manner that avoidsflaring or expanding the tube ends. In one embodiment, local deformationof the tube end surfaces or tube sheet surfaces and galling betweenadjacent surfaces can provide sufficient retention to keep the installedtube ends from being dislodged under a large axially applied load thatoccurs during the FSW process.

One example of a local deformation is a knurled exterior tube surface atthe tube ends. Other examples of achieving an interference fit include,but are not limited to, the use of tubing with external features such ascorrugations and low-fin designs, or tubes with exterior raised featuresthat become deformed during installation to effectively locking the tubeends in place in an interference fit.

In one embodiment, a process of connecting a tube to a tube sheetincludes inserting an end of the tube into a hole in the tube sheet withan interference fit between the tube end and the tube sheet that locksthe tube end into the tube sheet without flaring or expanding the end ofthe tube adjacent to the tube sheet. The tube end is then FSW to thetube sheet.

In another embodiment, a heat exchanger includes a first tube sheethaving a plurality of holes, an inner side and an outer side. Aplurality of tubes have first ends disposed in a corresponding one ofthe holes in the first tube sheet, with an interference fit between eachfirst end and the first tube sheet, and the first end of each tube isnot flared or expanded. The first end of each tube is FSW to the firsttube sheet at the outer side of the first tube sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the layout of a conventional OTEC powergeneration system in which the described heat exchanger can be utilizedin one embodiment.

FIG. 2 is a cross-sectional view of a shell and tube heat exchangeraccording to one embodiment described herein.

FIG. 3A is an end view of one of the tube sheets of the heat exchangerin FIG. 2.

FIG. 3B is close-up view of the region A-A in FIG. 3A.

FIG. 4 illustrates one of the tubes according to one exemplaryembodiment.

FIG. 5 illustrates the end of a tube showing one exemplary embodimentfor creating an interference fit.

FIG. 6 illustrates the end of a tube showing another exemplaryembodiment for creating an interference fit.

FIG. 7 is a cross-sectional view showing details of the joint betweenone of the tubes and the tube sheet prior to FSW.

FIG. 8 illustrates another exemplary embodiment for creating aninterference fit.

FIGS. 9A and 9B illustrate the embodiment of FIG. 8 used with the tubesshown in FIGS. 5 and 6 respectively.

FIG. 10 is a cross-sectional view through one of the holes in a tubesheet illustrating the embodiment of FIG. 8 installed in the hole of thetube sheet.

DETAILED DESCRIPTION

This description describes an approach to producing shell and tube heatexchangers, where prior to FSW of the tubes to the tube sheet, aninterference fit is used to lock the ends of the tubes into the tubesheet without flaring or expanding the tube ends. A FSW process is thenused to weld the ends of the tubes to the tube sheet.

The resulting shell and tube heat exchanger can be used in any heatexchange application, whether on land or on or in water. However, thedescribed shell and tube heat exchanger has particular benefits in asalt water environment as the shell and tube heat exchanger eliminatescrevices or local deformations near the ends of the tubes that can formsites for preferential crevice corrosion.

To help explain the inventive concepts, a specific application of theshell and tube heat exchanger in an OTEC system will be described.However, it is to be realized that the shell and tube heat exchanger isnot limited to use in an OTEC system, but instead can be used in anyheat exchange application.

In addition, the inventive concepts are not limited to use in a shelland tube heat exchanger, but instead can be used in any heat exchangerapplication where it is desirable to connect a tube to a supportingstructure using FSW, including but not limited to, in a manner thateliminates crevices or local deformations near the ends of the tubesthat can form sites for preferential crevice corrosion. Therefore, theterm tube sheet as used herein and in the claims is intended to broadlyencompass any supporting structure to which a tube is to be secured,unless otherwise indicated.

FIG. 1 is a schematic diagram of the layout of a conventional OTEC powergeneration system 100. The overall construction and operation of an OTECsystem is well known to those of ordinary skill in the art. The OTECsystem 100 can be deployed in any suitable body of water such as anocean, sea, a salt or fresh water lake, etc.

In this embodiment, the system 100 includes an offshore platform 102, aturbogenerator 104, a closed-loop conduit 106, an evaporator 110-1, acondenser 110-2, a hull 112, a plurality of pumps 114, 116, and 124, andfluid conduits 120, 122, 128, and 130. The closed-loop conduit 106 is aconduit for conveying working fluid 108 through the evaporator 110-1,the condenser 110-2, and the turbogenerator 104.

The evaporator 110-1 can be a shell-and-tube heat exchanger that isconfigured to transfer heat from warm seawater at surface region andworking fluid 108 thereby inducing the working fluid to vaporize.

The condenser 110-2 can also be a shell-and-tube heat exchanger that isconfigured to transfer heat from vaporized working fluid 108 to coldseawater from the deep-water region thereby inducing condensation ofvaporized working fluid 108 back into liquid form.

FIG. 2 illustrates a shell and tube heat exchanger 110 that can be usedfor the evaporator 110-1 and/or the condenser 110-2. In this example,the heat exchanger 110 includes a shell 202, a first fluid inlet 204, aninput manifold 206, an output manifold 208, a first fluid outlet 210, asecondary fluid port 212, a secondary fluid port 214, tubes 216 thatform a tube bundle, first and second tube sheets 220 and baffles 224. Aswould be understood by a person of ordinary skill in the art, the heatexchanger 110 provides heat exchange between a first fluid that flowsthrough the tubes 216 and a secondary fluid that flows through the shell202 across the outer surface of each of the tubes 216. In oneembodiment, the first fluid inlet 204 and the first fluid outlet are forseawater that flows through the tubes 216. In the case of evaporators,the fluid port 212 can be an outlet port for working fluid while thefluid port 214 can be an inlet port for the working fluid. In the caseof condensers, the fluid port 212 can be an inlet port for working fluidwhile the fluid port 214 can be an outlet port for the working fluid.

Further information of the construction of the heat exchanger 110 isdisclosed in U.S. Pat. No. 8,439,250, which is incorporated herein byreference in its entirety.

With reference to FIGS. 3A and 3B, each of the tube sheets 220 is amechanically rigid plate that includes a plurality of holes 218 thatextend through the tube sheet 220 from an inner side surface 222 (FIG.7) that faces the interior chamber of the heat exchanger 110 to an outerside surface 223 (FIG. 7) that faces the respective manifold 206, 208.Each hole 218 is illustrated as being circular with a diameter D1.Likewise, the tube sheets 220 are illustrated as being circular but canhave any shape suitable for use in a heat exchanger.

FIG. 4 illustrates an example of one of the tubes 216. Each tube 216includes a first end 230 and a second end 232, a central fluidpassageway 234, and a length L between the first end and the second end.In the illustrated example, the tubes 216 are shown as being cylindricalin shape with an inner diameter ID1 and an outer diameter OD1. However,the tubes 216 and the holes 218 in the tube sheets 220 can have anycomplementary shapes, such as rectangular or triangular.

The tubes 216 and the tube sheets 220 can be made of any metals commonlyused in shell and tube heat exchanger assemblies including, but notlimited to, aluminum, carbon steel, stainless steel, titanium, copper orother metals and alloys thereof.

The ends 230, 232 of the tubes 216 are joined to the tube sheets 220 atthe holes 218 using a FSW process. FSW is a well-known method forjoining two elements of the same or dissimilar material. FSW employs arotating probe that is forced into the interface between the twoelements. The immense friction between the probe and materials causesmaterial in the immediate vicinity of the probe to heat up totemperatures below its melting point. This softens the adjoiningsections, but because the material remains in a solid state, itsoriginal material properties are retained. Movement of the probe alongthe weld line forces the softened material from the two pieces towardsthe trailing edge causing the adjacent regions to fuse, thereby forminga weld.

As discussed above, an interference fit is used to lock the ends 230,232 of the tubes 216 into the holes of the tube sheets 220 withoutflaring or expanding the tube ends. The FSW process is then used to weldthe ends of the tubes to the tube sheet.

Numerous possibilities exist for creating a suitable friction fit, andany technique that creates a friction fit without flaring or expandingthe tube ends can be used. The exterior surface of the tube can bemodified, the interior surface of the holes 218 can be modified, or acombination of modifying the tube exterior surface and the hole interiorsurface can be used.

FIG. 5 illustrates an end of a tube 250 that can be used, where theexterior surface of the tube 250 adjacent to each end is knurled 252 aswell as having a non-knurled, smooth portion 254 between the knurledportion and the terminal end of the tube. A suitable tube with a knurledexternal surface is available from Energy Transfer, Inc. of Minerva,Ohio.

In this example of FIG. 5, the outer diameter OD1 of the tube 250 withthe knurled exterior is substantially equal to or only slightly lessthan the diameter D1 of the holes 218, such that the end of the tube 250can be inserted into the hole 218 with an interference fit therebetween.The interference fit should be sufficient to react against the loadsthat are applied during the FSW process, thereby locking the end of thetube in place and prevent axial and rotational movement of the tubeduring the FSW process.

In addition, in the example of FIG. 5, the inner diameter ID1 of thetube 250 both before and after FSW is constant from one end to theother. The outer diameter OD1 is also substantially constant between theends of the tube both before and after FSW.

In one embodiment, when the tube end is inserted into the hole 218, someof the smooth portion 254 is located in the hole along with some of theknurled portion 252.

FIG. 6 illustrates another embodiment of a tube 260 that includes finsor corrugations 262 (that are similar to threads) on the exteriorsurface for creating the interference fit. A suitable tube with thisconstruction is available from Energy Transfer, Inc. of Minerva, Ohio.An optional press-fit collar 264 having deflectable tabs 266 separatedby gaps can also be disposed around the fins 262 to enhance theinterference fit. The tabs 266 deflect inwardly to lock onto the fins262. Alternatively, the collar 264 can be disposed within the hole 218of the tube sheet 220 with the tube sliding into the hole in onedirection but cannot be pulled out because the tabs engage with thefins. In another alternative embodiment, as shown in FIG. 7, the collar264 can fit in a hole in the baffle 224 or otherwise engage with thebaffle 224 for load support.

FIG. 8 illustrates an example of a press-fit collar 270 that can bedisposed around a tube. The collar 270 can have deflectable tabs 272that are similar to the tabs 266 in FIG. 6. The tabs 272 are designed todeflect radially inwardly to create an interference fit with theenhanced (for example, the knurls 252 in FIG. 5, the fins 262 in FIG. 6,or the like) outer surface of the tube. The collar 270 can also haveexternal threads 274 on a portion of the collar without the deflectabletabs 272. The external threads 274 are intended to thread intocorresponding threads 276 (see FIG. 10) formed in a receiving hole 278in the side surface 222 of the tube sheet 220 or on either side of ahole formed in the baffle 224.

As is evident from FIG. 10, the receiving hole 278 has a larger diameterthan the remainder of the hole 218 that the tube 250, 260 forms afriction fit with. The interior surface of the hole 278 is formed withthe threads 276 that engage with the threads 274 on the exterior of thecollar 270.

FIG. 9A depicts an example of the tube 250 and collar 270 assemblysecured within a hole in the tube sheet 220 (the tube sheet 220 is onlypartially shown with one hole). FIG. 9B depicts an example of the tube260 and collar 270 assembly secured within a hole in the tube sheet 220(the tube sheet 220 is only partially shown with one hole). In oneembodiment, the collar 270 is initially press fit onto either theknurled tube 250 or the tube 270 with the fins. The tube and collarassembly are then threaded into the threaded hole 278 of the tube sheetor the baffle.

To assist in threading the collar 270 into the receiving hole 278 of thetube sheet 220, the collar 270 can also have an extended cylindricalfeature 280 projecting from the threaded portion that has cutouts 282,flat faces or other wrench or tool engagement structure so that aspanner wrench or other tool can be used to rotate the collar 270 totighten the collar 270 and tube assembly into the threaded hole 278and/or unthread the collar from the hole.

The threading features on the collar 270 and on the receiving hole 278allows assembly personnel to precisely screw/fasten the collar-and-tubeassembly into the tube sheet 220 such that the tube end is flush withthe outer surface 223 of the tube sheet. This precision threadingfeature solves the issue of manufacturing tolerances that cause fit-upvariances using other press-fit methods

Another example of creating an interference fit could be a lock andkey-type where, similar to FIG. 6, a swaged type of key could be pressedonto the tube that could have grooves to engage the bottom side of thetube sheet 220 or tube baffle 224. In another embodiment, instead of akey, a tapered fin or wedge could be integrally formed on the tube thatlocks against the inner surface of the tube sheet hole to provide theinterference fit.

In another embodiment, the back side of the tube sheet 220 could betapped with a thread pattern that matches the external fins or threads262 on the tube 260 in FIG. 6. The tube 260 could then be threaded intothe backside of the tube sheet 220 providing the necessary resistancefor the friction stir weld.

Once the tube ends are inserted into the holes of the tube sheet. FSWcan then be used to weld the tube ends to the tube sheet.

For the second ends of the tubes and the second tube sheet, the secondends may be fixed to the second tube sheet using a conventional FSWprocess like that described in U.S. Pat. No. 8,439,250 where the secondends of the tubes are immobilized relative to the second tube sheet byexpanding the second ends of the tubes, followed by FSW to weld thesecond ends of the tubes to the tube sheet.

Example Process

The following describes a sample process used to secure aluminum tubesto an aluminum tube sheet. This exemplary process is applicable for onlyone side (i.e. one of the tube sheets) of the heat exchanger and not forboth tube sheets, since the second tube sheet cannot be accessed in thesame manner as the first tube sheet.

In this example, the tubes were 1 inch NPS schedule 40 coarse-knurledtubes with 3 inches of smooth tube on the end (similar to the embodimentof FIG. 5), 12 inches in length. The tube sheet was a 2 inch thickaluminum tube sheet with holes for 7 tubes. In this exemplary process, abacking anvil/force to support the tubes is not used; only the tubesheet is supported.

1. The tube sheet was supported over the FSW anvil by placing aluminumblocks under opposing sides of the tube sheet, without blocking the tubeholes. The height of the blocks determined how much of the tube can passthrough the tube sheet hole before it is stopped at the anvil.

2. The exterior of the tubes and the interior of the tube sheet holeswere cleaned with isopropyl alcohol, acetone, or other cleaner.

3. The tubes were then inserted into the tube sheet holes. The oppositetube ends were then struck using a rubber mallet to drive the tubesthrough the tube sheet until the tubes touch the anvil below the tubesheet. Approximately 0.5 inch of the smooth tube section 254 remained inthe tube sheet hole for proper welding, with the remainder of the tubewithin the hole being the knurled section 252.

4. By now, the tubes should be near-galled and should not move in thetube sheet. On the weld side (i.e. the outer side of the tube sheet),the tubes were cut so that the tube ends are flush with the outersurface of the tube sheet.

5. The weld surface and the interior of the tubes were then cleaned.

6. The tubes are then deburred and the tubes and the tube sheet are thenFSW. There should be no axial movement or spinning observed in the tubesas the weld proceeds.

FIG. 7 illustrates an example of the tube 216 inserted into one of theholes of the tube sheet 220, with the tube cut so as to be flush withthe outer surface 223 and ready for FSW. As seen in FIG. 7, there is noflaring or expanding of the tube end prior to FSW. Instead, the innerdiameter and outer diameter of the tube remain constant. Therefore, thedescribed method does not introduce any crevice or other localdeformation near the tube end that could act as a preferential site forcorrosion.

The examples disclosed in this application are to be considered in allrespects as illustrative and not limitative. The scope of the inventionis indicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A process of connecting a tube to a tube sheet,comprising: inserting a tube end of the tube into a tube sheet hole ofthe tube sheet with an interference fit between the tube end and thetube sheet that locks the tube end into the tube sheet without flaringor expanding any cross-sectional dimension of the tube end adjacent tothe tube sheet; and friction stir welding the tube to the tube sheet. 2.The process of claim 1, wherein the interference fit is created bymodifying an exterior surface of the tube, modifying an interior surfaceof the tube sheet hole, or both modifying the exterior surface of thetube and modifying the interior surface of the tube sheet hole.
 3. Theprocess of claim 2, wherein the interference fit is created by modifyingthe exterior surface of the tube to include knurls or fins.
 4. Theprocess of claim 2, wherein the interference fit is created by modifyingthe exterior surface of the tube to include a plurality of corrugations.5. The process of claim 2, wherein the interference fit is created bymodifying the exterior surface of the tube to include a thread.
 6. Theprocess of claim 1, wherein the interference fit is created by a taperedfin formed on the tube end.
 7. The process of claim 1, furthercomprising installing a collar around the tube, and at least partiallyfitting the collar into the tube sheet hole or into a hole in a bafflethat is separate from the tube sheet.
 8. The process of claim 7, whereinthe collar includes a plurality of deflectable tabs that can deflectradially inward to engage with an exterior surface of the tube.
 9. Theprocess of claim 8, wherein the collar further includes externalthreads, the tube sheet hole or the hole in the baffle includes interiorthreads on an interior surface of the hole in the baffle, the processfurther comprising threading the collar into the tube sheet hole or intothe hole in the baffle so that the external threads on the collar areengaged with the interior threads.
 10. The process of claim 9, whereinthe collar further includes a tool engagement structure that facilitatesrotation of the collar relative to the tube sheet hole or the hole inthe baffle to thread the collar into or out of the tube sheet hole orthe hole in the baffle, and threading the collar into the tube sheethole or into the hole in the baffle using a tool engaged with the toolengagement structure.
 11. The process of claim 10, further comprisingthreading the collar into the tube sheet hole or the hole in the baffleuntil the tube end is flush with one face of the tube sheet.
 12. Theprocess of claim 1, wherein the tube and the tube sheet form part of aheat exchanger.
 13. The process of claim 1, wherein the tube end and thetube sheet hole are circular.
 14. A process of connecting a tube to atube sheet, comprising: inserting a tube end of the tube into a tubesheet hole of the tube sheet with an interference fit between the tubeend and the tube sheet that locks the tube end into the tube sheetwithout flaring or expanding the tube end adjacent to the tube sheet;and friction stir welding the tube to the tube sheet; wherein theinterference fit is created by modifying an exterior surface of thetube, modifying an interior surface of the tube sheet hole, or bothmodifying the exterior surface of the tube and modifying the interiorsurface of the tube sheet hole; wherein the exterior surface of the tubehas a modified exterior surface portion that creates the interferencefit with the tube sheet hole, and has a smooth exterior surface portionbetween the modified exterior surface portion and the tube end, andwherein: inserting the tube end of the tube into the tube sheet hole ofthe tube sheet with the interference fit between the tube end and thetube sheet that locks the tube end into the tube sheet without flaringor expanding any cross-sectional dimension of the tube end adjacent tothe tube sheet further comprises: inserting the tube end of the tubeinto the tube sheet hole of the tube sheet with the interference fitbetween the tube end and the tube sheet that locks the tube end into thetube sheet without flaring or expanding any cross-sectional dimension ofthe tube end adjacent to the tube sheet such that a portion of themodified exterior surface portion is within the tube sheet hole and aportion of the smooth exterior surface portion is within the tube sheethole.
 15. The process of claim 14, wherein the interference fit iscreated by modifying the exterior surface of the tube to include knurlsor fins.
 16. The process of claim 14, wherein the interference fit iscreated by modifying the exterior surface of the tube to include aplurality of corrugations.
 17. The process of claim 14, wherein theinterference fit is created by modifying the exterior surface of thetube to include a thread.
 18. The process of claim 14, wherein theinterference fit is created by a tapered fin formed on the tube end. 19.The process of claim 14, further comprising installing a collar aroundthe tube, and at least partially fitting the collar into the tube sheethole or into a hole in a baffle that is separate from the tube sheet.20. The process of claim 14, wherein the tube and the tube sheet formpart of a heat exchanger.